Flat panel displays are gaining popularity in a wide range of electronic devices. Common types of flat panel displays include active matrix displays and passive matrix displays. Each pixel in an active matrix display panel is driven by active driving circuitry, while each pixel in a passive matrix display panel does not use such driving circuitry. High-resolution color display panels, such as modern computer displays, smart phones and televisions typically use an active matrix display panel structure for better image quality.
One kind of display panel that is finding commercial application is an active matrix organic light emitting diode (AMOLED) display panel. FIG. 1 is a top view illustration of a top emission AMOLED display panel. FIG. 2 is a cross-sectional side view illustration of FIG. 1 taken along line X-X in the pixel area 104 and line Y-Y crossing the ground ring 116 in the non-pixel area. The AMOLED display panel 100 illustrated in FIGS. 1-2 generally includes a thin film transistor (TFT) substrate 102 supporting a pixel area 104 and non-pixel area outside of the pixel area 102. A TFT substrate 102 is also referred to as a backplane. A TFT substrate which has been further processed to additionally include the pixel area and non-pixel area is also often referred to as a backplane. Two primary TFT substrate technologies used in AMOLEDs include polycrystalline silicon (poly-Si) and amorphous silicon (a-Si). These technologies offer the potential for fabricating the active matrix backplanes at low temperatures (below 200° C.) directly onto flexible plastic substrates for producing flexible AMOLED displays. The pixel area 104 generally includes pixels 106 and subpixels 108 arranged in a matrix, and a set of TFTs and capacitors connected to each subpixel for driving and switching the subpixels. The non-pixel area generally includes a data driver circuit 110 connected to a data line of each subpixel to enable data signals (Vdata) to be transmitted to the subpixels, a scan driver circuit 112 connected to scan lines of the subpixels to enable scan signals (Vscan) to be transmitted to the subpixels, a power supply line 114 to transmit a power signal (Vdd) to the TFTs, and a ground ring 116 to transmit a ground signal (Vss) to the array of subpixels. As shown, the data driver circuit, scan driver circuit, power supply line, and ground ring are all connected to a flexible circuit board (FCB) 113 which includes a power source for supplying power to the power supply line 114 and a power source ground line electrically connected to the ground ring 116.
In the exemplary AMOLED backplane configuration an organic thin film 120 and top electrode 118 are deposited over every subpixel 108 in the pixel area 104. The organic thin film 120 may include multiple layers such as a hole injection layer, hole transport layer, light emitting layer, electron transport layer, and electron injection layer. The multiple layers of the organic thin film 120 are typically formed over the entire pixel area 104, however, the light emitting layer is often deposited with aid of a shadow mask only within the subpixel openings 127 and on the bottom electrode layer 124 corresponding to the emission area for the array of subpixels 108. A top electrode layer 118 is then deposited over the organic thin film within both the pixel area 104 and also within the non-pixel area so that the top electrode 118 layer overlaps the ground ring 116 in the in order to transfer the ground signal to the array of subpixels. In this manner, each of the subpixels 108 can be individually addressed with the corresponding underlying TFT circuitry while a uniform ground signal is supplied to the top of the pixel area 104.
In the particular implementation illustrated, the TFT substrate 102 includes a switching transistor T1 connected to a data line 111 from the data driver circuit 110 and a driving transistor T2 connected to a power line 115 connected to the power supply line 114. The gate of the switching transistor T1 may also be connected to a scan line (not illustrated) from the scan driver circuit 112. A planarization layer 122 is formed over the TFT substrate, and openings are formed to expose the TFT working circuitry. As illustrated, a bottom electrode layer 124 is formed on the planarization layer in electrical connection with the TFT circuitry. Following the formation of the electrode layer a pixel defining layer 125 is formed including an array of subpixel openings 127 corresponding to the emission area for the array of subpixels 108, followed by deposition of the organic layer 120 and top electrode layer 118 over the patterned pixel defining layer, and within subpixel openings 127 of the patterned pixel defining layer 125. The top electrode layer 118 additionally is formed in the non-pixel area and in electrical connection with the ground ring 116.
The planarization layer 122 may function to prevent (or protect) the organic layer 120 and the bottom electrode layer 124 from shorting due to a step difference. Exemplary planarization layer 122 materials include benzocyclobutene (BCB) and acrylic. The pixel defining layer 125 can be formed of a material such as polyimide. The bottom electrode 124 is commonly formed on indium tin oxide (ITO), ITO/Ag, ITO/Ag/ITO, ITO/Ag/indium zinc oxide (IZO), or ITO/Ag alloy/ITO. The top electrode layer 118 is formed of a transparent material such as ITO for top emission.
While AMOLED display panels generally consume less power than liquid crystal display (LCD) panels, an AMOLED display panel can still be the dominant power consumer in battery-operated devices. To extend battery life, it is necessary to reduce the power consumption of the display panel.